Iron-based powder compositions for powder metallurgy have been produced generally by mixing an iron powder as the base material, and an alloying powder such as copper powders, graphite powders, and iron phosphide powders, and, if necessary, a machinability-improving powder, and a lubricant such as zinc stearate, aluminum stearate, and lead stearate. The lubricant has been selected in consideration of its mixability with the iron powder and its removability in the sintering process.
In recent years, in powder metallurgy, sintered members are demanded to have higher strength. To meet the demand, a "warm compaction technique" has been developed in which powdery material filled in a metal die is compacted with heating at a certain temperature to obtain a compact having a higher density and a higher strength (See, for example, Japanese Patent Application Laid-Open Gazette (Kokai) No. Hei.2-156002, Japanese Patent Publication (Kokoku) No. Hei.7-103404, U.S. Pat. No. 5,256,185, and U.S. Pat. No. 5,368,630). The lubricant added to the iron powder in the warm compaction technique should have lubricity in the compaction process in addition to the above required properties. This lubricity is important to improve the compactibility by reducing frictional resistance between the iron powder particles and between the metal die and the formed compact by melting a part or the entire of the lubricant and dispersing it uniformly throughout the iron powder particle interspace. However, a conventional powder mixture is liable to cause particle segregation of an alloying powder or other additive disadvantageously. A powder mixture generally contains powder particles having various particle sizes, various particle shapes, and different particle densities, so that segregation tends to occur during transportation after the mixing, on charging into or discharging from a hopper, or during compacting.
For example, a mixture of iron-based powder and graphite powder is known to undergo particle segregation during truck transportation by vibration in a transporting vessel to separate graphite particles on the powder surface. A powder composition charged into a hopper undergoes segregation during movement within the hopper, causing variation of graphite powder content in the discharged powder composition from the initial stage to the end stage of the discharge. The final sintered articles produced from the segregated nonuniform powder composition are liable to vary in chemical composition, dimension, and strength, which can make the products inferior. The graphite powder or an additive, which is usually fine powdery, increases the specific surface area of the powder composition to lower the flowability of the composition. The lower flowability of the composition decreases the speed of filling the powder composition into a die cavity, lowering the compact production rate.
For preventing the segregation of the powder composition, addition of a binder is disclosed in Japanese Patent Application Laid-Open Gazette Nos. Sho.56-136901 and Sho.58-28321. However, a larger amount of addition of a binder to prevent the segregation in the powder composition poses another problem of fall of the flowability of the entire powder composition disadvantageously.
The inventors of the present invention disclosed use of a co-melted mixture of a metal soap or a wax and an oil as a binder in Japanese Patent Application Laid-Open Gazette Nos. Hei.1-165701 and Hei.2-47201. The disclosed binder reduces remarkably the segregation of the powder composition and the scattering of dust, and improves the flowability. However, this technique poses another problem of variation of the flowability of the powder composition with lapse of time owing to the above method of segregation prevention, namely the increase of the amount of the binder.
The inventors of the present invention disclosed use of a co-melted mixture of a high-melting oil and a metal soap as a binder in Japanese Patent Application Laid-Open Gazette No. Hei.2-57602. This technique reduces deterioration with time of the properties of the co-melted mixture and deterioration with time of flowability of the powder composition. This technique, however, poses still another problem such that the apparent density of the powder composition changes because a high-melting saturated fatty acid in a solid state and a metal soap are mixed with the iron-based powder. To solve this problem, the inventors of the present invention disclosed, in Japanese Patent Application Laid-Open Gazette No. Hei.3-162502, a method in which the surface of the iron-based powder particles is coated with a fatty acid, an alloying powder or a like additive is allowed to adhere thereto through a co-melted mixture of a fatty acid and a metal soap, and then a metal soap is added onto the outer surface thereof.
The above techniques disclosed in Japanese Patent Application Laid-Open Gazette Nos. Hei.2-57602 and Hei.3-162502 solve the problems of segregation in the powder composition and generation of dust to a considerable extent. With this technique, however, the flowability of the powder composition is insufficient: especially the flowability in "warm compaction" in which the powder composition heated to about 150.degree. C. is filled in a hot die and is compacted. Further, the improvements of compactibility of the powder composition in warm compaction disclosed in Japanese Patent Application Laid-Open Gazette Nos. Hei.2-156002, and Hei.7-103404, U.S. Pat. No. 5,256,185, and U.S. Pat. No. 5,368,630 mentioned above are not sufficient in the flowability of the powder composition in warm compaction owing to liquid bridge formation by a low-melting lubricant component between particles. The insufficient flowability not only reduces the productivity of the compacts but also causes variation of the density of the compacts and variation of the properties of the final sintered products. Furthermore, the warm compaction technique disclosed in above Japanese Patent Application Laid-Open Gazette No. Hei.2-156002, etc. enables production of iron-based compact having high density and high strength, but requires stronger ejection force for removal of the compact from the die and is liable to cause scratches on the compact surface or to shorten the life of the die.
The present invention intends to provide an iron-based powder composition for powder metallurgy excellent in flowability and compactibility in comparison with conventional ones at room temperature and in warm compaction, and intends also to provide a process for producing the powder composition, and a process for producing a compact having a higher density and a higher strength.